Christopher Columbus gained power for Spain by "discovering" the New World and claiming it for Spain. Because of his actions, Spain was able to claim, and then to conquer, a great deal of territory in what is now Mexico and Central and South America.
Spain profited greatly from its colonies in the Americas. The mines of Mexico and Peru, in particular, gave Spain huge amounts of money that it could use to fund its military and increase its power. This helped Spain in its European wars, for example. Because Columbus's "discovery" made this possible, we can say that he gained power for Spain.